Optical information communication systems use optical modulators. Therefore, it is useful for obtaining effective optical information communication systems to understand the characteristics of the optical modulators. Examples of the parameters for defining the characteristics of the optical modulators include insertion loss, modulation index, half-wavelength voltage (Vπ), optical band, ON/OFF extinction ratio, polarized wave extinction ratio, and chirp parameter. Methods for evaluating the characteristics of optical modulators have been researched and some evaluating methods have been reported.
Japanese Patent No. 3538619 discloses an invention for measuring a power spectrum of an MZ optical modulator and obtaining a modulation index using the measured power spectrum.
Japanese Patent No. 3866082 discloses an invention for obtaining a half-wavelength voltage and a chirp parameter of an optical modulator based on a spectrum distribution of an MZ optical modulator.
The methods for evaluating the characteristics of an optical modulator disclosed in the above two documents are excellent evaluating methods. However, it is desired to obtain a method for evaluating the characteristics of an optical modulator with more accurately.
In particular, in recent years, an optical modulator including a plurality of MZ interferometers has been developed. It is desired that characteristics of individual MZ interferometers in such an optical modulator are evaluated. On the other hand, in the conventional methods for evaluating the characteristics of an optical modulator, since there is generated a noise due to an MZ interferometer whose physicality is not evaluated, suitable evaluation cannot be made.    Patent Document 1: Japanese Patent No. 3538619    Patent Document 2: Japanese Patent No. 3866082